Lithium nuclear microprobe

ABSTRACT

A beam of lithium ions (7Li) is accelerated and bombarded against the surface of a target material containing hydrogen atoms at sufficient energy to stimulate an 1H (7Li, gamma )8Be radiative capture reaction and the intensity of the resulting emission of capture gamma rays is directly proportional to the number of hydrogen nuclei (protons) in the target material at a particular depth. The 1H (7Li, gamma )8Be reaction occurs only within a very narrow range of relative velocities between the lithium and hydrogen nuclei and, consequently, within a very narrow region inside the target material, permitting the concentration of hydrogen in solids to be measured at selected depths by varying the bombarding energy of the lithium ions.

United States Patent [1 1 Padawer Jan. 9, 1973 [54] LITHIUM NUCLEAR MICROPROBE OTHER PUBLICATIONS Qarlson et a], High-Enggy Gamma Radiation from Lithium Bombardment of Be B11, and c Accewm ran Physical Review, Vol. 136, No. 3B, pp. 630-636. 7

Primary Examiner-James W. Lawrence Assistant Examiner-Davis L. Willis Attorney-Morgan, Finnegan, Durham & Pine [57] ABSTRACT A beam of lithium ions ("Li) is accelerated and bombarded against the surface of a target material containing hydrogen atoms at sufficient energy to stimulate an H (Li, y)Be radiative capture reaction and the intensity of the resulting emission of capture gamma rays is directly proportional to the number of hydrogen nuclei (protons) in the target material at a particular depth. The H (Li, 'y)*Be reaction occurs only within a very narrow range of relative velocities between the lithium and hydrogen nuclei and, consequently, within a very narrow region inside the target material, permitting the concentration of hydrogen in solids to be measured at selected depths by varying the bombarding energy of the lithium ions.

3 Claims, 2 Drawing Figures RAY Den-cm PATENTEDJAN 9 ma l/ MAE AR l2 Aacswmron FIG. I

01s DEPTH mm Sumac; (/10, 0005/.)

5 i=5 .3 Egg 2 INVENTOR. GERALD MPADAWER Mz Armmvsx;

LITHIUM NUCLEAR MICROPROBE BACKGROUND AND OBJECTS OF THE INVENTION l Field of the Invention This invention relates generally to materials analysis, and relates more particularly to a novel and improved method for measuring the concentration of hydrogen in solid materials at selected depths near the surface of the material, and within a very narrow region at any particular depth.

2. Description of the Prior Art A major area of scientific investigation is in surface contamination of solid materials and the effect of the contaminants on such properties as ductility, static strength, and fatigue life of the material. In conducting these investigations, it is important to obtain a measurement of the concentration gradient, or density profile, of the contaminant within the material. This information can then be used to correlate processing variables in order to devise methods by which the deleterious effects of the contamination can be minimized or eliminated.

A particularly critical surface contamination problem is the embrittlement of metals resulting from contamination by hydrogen, which causes structural failures. It is believed likely that hydrogen-embrittlement-related structural failures are initiated by strong hydrogen concentration gradients, but it has not heretofore been possible to obtain a measurement of these gradients.

Heretofore it has been feasible to obtain only bulk assays of the hydrogen content in a given sample. These measurements average the hydrogen content over a large volume and therefore are incapable of yielding information concerning hydrogen concentration within a small depth increment. Consequently, heretofore it has not been possible to obtain a measurement of hydrogen concentration gradients, or density profiles, in a solid material as a function of depth.

3. Objects of the Invention It is therefore an object of this invention to provide a novel and improved method for measuring the content of hydrogen near the surface of a solid material.

Another object of this invention is to provide a novel and improved method for measuring the concentration gradient of hydrogen near the surface of a solid material as a function of depth.

Another object of this invention is to provide a novel and improved method for measuring the concentration of hydrogen in solid materials at selected depths near the surface of the material, and within a very narrow region at any particular depth.

Objects and advantages of the invention are set forth in part herein and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the processes and combinations pointed out in the appended claims.

The invention consists in the novel steps, processes, arrangements, combinations and improvements herein shown and described.

SUMMARY OF THE INVENTION Briefly described, the method of the present invention for measuring the concentration gradient of hydrogen near the surface of a solid material comprises: accelerating a beam of lithium ions (Li) and directing the beam against the surface of a solid material containing hydrogen atoms at sufficient energy to stimulate an H (Li, 7 )Be radiative capture reaction. The intensity of the resulting emission of capture gamma rays is then suitably measured and is directly proportional to the number of hydrogen nuclei (protons) in the target material at a particular depth.

It will be apparent from the foregoing general description that the objects of the invention specifically enumerated herein are accomplished by the invention as here embodied.

The well known I .i (p, 'y Be radiative capture reaction occurs only within a very narrow range of relative velocities between the lithium and hydrogen nuclei. Consequently, by interchanging the customary roles of the hydrogen nuclei (protons) and the lithium nuclei, so that the lithium atom becomes the projectile and the hydrogen atom becomes the target, i.e., H (Li, 'y )Be it is possible to determine the concentration of hydrogen within a very narrow depth increment inside a solid material, which is that discrete region within which the relative velocities of the lithium and hydrogen nuclei will stimulate the H (Li, 'y )Be reaction. Since the energy of the lithium atoms is degraded by collisions as the beam passes through the target material, the concentration of hydrogen in a solid may be measured at selected depths by varying the bombarding energy of the lithium ions. It is therefore possible to ascertain the concentration gradient of the hydrogen as a function of depth.

It has been found that the method of the invention is highly advantageous for several other reasons. For one, the resonance energy level at which the highly excited Be compound nucleus is formed can be easily attained, and there are no other nearby resonances in Be. The gamma rays which are emitted upon the de-excitation of Be are, in turn, extremely energetic and hence are easily identified and distinguished from background gamma rays. Also, there is a low capture probability for the lithium nuclei by nuclei in the target material other than hydrogen. Also, a suitable source of lithium ions is readily available. Finally, the method of the invention has been found to be non-destructive, permitting continued examination of a material under investigation.

It will be understood that the foregoing general description and the following detailed description as well are exemplary and explanatory of the invention but are not restrictive thereof.

The accompanying drawings, referred to herein and constituting a part hereof, illustrate one embodiment of the invention, and together with the description, serve to explain the principles of the invention.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view, illustrating a greatly enlarged fragmentary portion of a material containing hydrogen atoms being bombarded by a beam of accelerated lithium ions and subsequent measurement of the intensity of the capture gamma ray emission at the depth within the target material where the energy of the lithium ions stimulates an H ('Li, y )Be radiative capture reaction, in accordance with the method of the present invention; and

FIG. 2 is a graph whose coordinates are depth into the target material surface and quantity of gamma ray emission, providing an illustrative profile of the concentration of hydrogen atoms in the target material shown in FIG. 1 as a function of depth within that material.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now more particularly to the embodiment of the method of the invention shown in FIG. 1 of the accompanying drawing, there is illustrated a greatly enlarged fragmentary portion near the surface of a sample of a solid target" material, such as metal, indicated generally by reference numeral 10, containing hydrogen atoms (I-I) 11.

In accordance with the invention, the concentration of the hydrogen atoms 11 inside the solid target material is determined by bombarding the target material with lithium ions ('Li) accelerated to a sufficient energy to stimulate an H (Li, 7 )Be radiative capture reaction and measuring the intensity of the resulting capture gammaray emission.

Thus, in accordance with the invention, the customary roles of the hydrogen nuclei (protons) and the lithium nuclei are interchanged, whereby the lithium atom becomes the projectile and the hydrogen atom becomes the target. To this end, an accelerator 12, suitably equipped with an internal source capable of producing lithium ions ("Li) accelerates those ions to a high velocity, and the ion beam 13 is directed against the surface 14 of the target material. The gamma rays producedasa result of the H ("'Li, 7 )Be radiative capture reaction upon the collision of a lithium atom with a hydrogen atom are diagrammatically illustrated at 15 and the intensity thereof may be measured by a suitable gamma ray detector 16, such as a scintillation crystal.

As previously mentioned, the well known "Li (p, 7 )Be radiative capture reaction is stimulated only within a very narrow range of relative velocities between the lithium nucleus and the hydrogen nucleus (proton), within which interval fusion of the lithium nucleus and the proton to form the compound nucleus Be occurs readily. This region of resonance corresponds to the formation of a highly excited, isolated sharp energy level in Be. This energy level de-excites promptly by the emission of a gamma ray which is extremely energetic, on the order of 15-18 MeV (million electron volts), and, hence, easily identifiable and distinguishable from the range of background gamma rays, very few of which exceed approximately 10 MeV.

In the Li (p, 'y )Be radiative capture reaction, wherein the proton is the projectile and the lithium is the stationary target, the resonance level, or energy of the proton required to form the compound nucleus Be, occurs at 0.44 MeV.

in the method of the present invention, where lithium is the projectile and the proton (H nucleus) is the stationary target, the minimum kinetic energy of the lithium ion required to obtain the same relative velocity, or resonance level, at which the compound nucleus Be is formed is seven (7) times greater, or- 3.1 MeV. The factor 7 is the mass ratio of the "Li nucleus to the proton, or H nucleus, and the requirement for this multiplication factor is demonstrated algebraically as follows:

Li ras on H =7E on 7, since where m mass v relative velocity E resonance energy.

In view of the foregoing, accelerator 13 ad vantageously comprises a single-stage or tandem Van de Graaff electrostatic generator capable of generating sufficient voltages to accelerate the lithium ions up to an energy of at least 3.1 MeV, and preferably accelerates the lithium ions up to energies on the order of v 9.5 MeV or greater.

Becuase the energy of the incident projectile beam is degraded by collisions as the beam passes through the target material, the appropriate projectile-target relative velocity is achieved only within a very narrow depth interval inside the target material. The intensity of the capture gamma rays produced at a particular bombarding energy of the lithium ions is thus directly proportional to the number of target atoms present in only this very narrow, defined region such as is shown at referencenumeral 17 in FIG. 1.

It will be apparent from the foregoing that, by varying the bombarding energy of the lithium ions from a minimum energy of 3.1 MeV to the maximum energy possible, the concentration of hydrogen in a solid material may be determined at selected depths and, consequently, the concentration gradient of hydrogen as a function of depth may be ascertained, as illustrated in FIG. 2. Thus, it has been found that of the maximum energy available with accelerators presently in existence, the hydrogen concentration of solid materials may be ascertained at depths up to 10 microns (1 micron 10" meter The width of the increment of depth at which the relative velocities between the lithium atoms and the hydrogen atoms are at the resonance level for stimulating the H (Li, 7 )Be radiative capture reaction may be calculated from the following formula:

dE/pdx is the stopping power of the projectile ion in the target medium, and is very nearly directly proportional to Z'projectile flv where Z atomic number and f a function of the relative velocity. Hence, at the same relative velocity, for lithium ions (Z 3; Z 9) in a given target material, 1

T; 7Li 1E 11; 5

From the previous mathematical treatment demonstrating the requirement for a multiplication factor of seven (7) for obtaining the resonance level where lithium is the projectile, it follows that,

AE'Im on 1 =7AEi1n on 71.1

whence the combining of these last two equations yields 7 AEi 7 Thus, where the target material is titanium p 4.5 gm/cm), mm;

dE m r p H 200 kev /mg cm from which A Ti NZ 12 kev.

I 7m on 1 9 4.5 gm./em. 200 10 kev./gm.-cm-

=10- cm., or 0.1 micron where 12 keV is the known line-width of the 0.44-MeV resonance for the "Li (p, 7 )Be reaction.

I The invention in its broader aspects is not limited to the specific embodiment herein shown and described but departures may be made therefrom without departing from the principles of the invention and without sacrificing its chief advantages.

What is claimed is: 1. A method for measuring the concentration of hydrogen atoms within a narrow increment of depth near the surface of a solid material, comprising the steps of:

accelerating lithium ions ('Li) to a high velocity sufficient to stimulate an H ('Li, 7 )Be radiative capture reaction;

directing the accelerated lithium ions in a beam against the surface of said solid material containing said hydrogen atoms; and

measuring the intensity of the capture gamma rays emitted as a result of said H (Li, y Be radiative capture reaction to thereby ascertain by direct proportion the number of hydrogen nuclei in said solid material at said narrow increment of depth where the relative velocities between said lithium and hydrogen nuclei stimulate said H (Li, 7 )Be radiative capture reaction.

2. A method as defined in'claim I, wherein said lithium ions ("Li) are accelerated to an energy of at least about 3.1 MeV 3. A method as defined in claim 1, including the steps varying the energy of said beam of accelerated lithium ions from a minimum energy of at least about 3.1 Me! to the maximum energy possible; and

measuring the intensity of the capture gamma rays emitted as a result of said H (Li, y )Be radiative capture reaction at each variation in said lithium ion beam energy to thereby ascertain a concentration gradient of the hydrogen atoms present in said solid material as a function of depth.

* IF it QERTEFEQT @F Q Q'HN Patent No. '3,7l0,ll3 Dated January '9, l973 Imam-tor) Gerald M. Padawer It: is certified that error appears in the above-ideotified patent and that said Letters Patentare hereby corrected as shown below:

lfitle page, column 1 should show the following information: 'Assignee: Grumman Aerospace Corporation,

Bethp'age, New York"; column 4, line should read a 1t the maximum".

46 "oi the maximum" Signed and sealed this 25th day of December 1973.

( A Atteso:

RENE D. IEG'I'MEYER Acting Gommissioner of Patent EDWARD M. FLETCHER,JR. Attesting Officer 

2. A method as defined in claim 1, wherein said lithium ions (7Li) are accelerated to an energy of at least about 3.1 MeV.
 3. A method as defined in claim 1, including the steps of: varying the eneRgy of said beam of accelerated lithium ions from a minimum energy of at least about 3.1 MeV to the maximum energy possible; and measuring the intensity of the capture gamma rays emitted as a result of said 1H (7Li, gamma )8Be radiative capture reaction at each variation in said lithium ion beam energy to thereby ascertain a concentration gradient of the hydrogen atoms present in said solid material as a function of depth. 